This invention relates to a method for oxidizing xylene derivatives. More particularly, the invention relates to a method for oxidizing a substrate comprising at least one halo-ortho-xylene in the presence of at least one metal catalyst, at least one solvent, and at least one promoter to provide a product comprising halo-phthalic acid or halo-phthalic anhydride. In one key embodiment the invention relates to a method for producing a product comprising 4-chlorophthalic acid or 4-chlorophthalic anhydride.
Methods for oxidizing ortho-xylene are known. For example, U.S. Pat. No. 3,402,184 describes oxidation of ortho-xylene in acetic acid solvent in the presence of a bromine promoter. U.S. Pat. Nos. 5,958,821, 5,981,420, and 6,020,522 describe oxidation of ortho-xylene in acetic acid solvent in the presence of a hydroxyimide promoter. Methods for preparing 4-chlorophthalic anhydride are also known. However, these methods typically involve aromatization of a Diels-Alder adduct of chloroprene and a maleic anhydride as in U.S. Pat. No. 5,322,954, or chlorination of phthalic acid as in Japanese patent applications 07258152 and 02129143. The latter chlorination process may produce polychlorinated biphenyls (PCBs). There is a need for a method for producing 4-chlorophthalic anhydride which does not involve handling toxic chloroprene or chlorine gas, and which does not produce PCBs.
In one embodiment the invention is a method for oxidizing a substrate comprising at least one halo-ortho-xylene which comprises combining the substrate in a solvent with at least one metal catalyst and heating in the presence of an oxygen source to produce a product mixture, wherein an effective amount of at least one promoter is added to the reaction mixture after the reaction has proceeded to an intermediate stage of conversion to product mixture.
In another embodiment the invention is a method for oxidizing a substrate comprising 4-chloro-ortho-xylene which comprises combining chloro-ortho-xylene in acetic acid solvent with at least one metal catalyst which is a metal compound comprising cobalt, and heating in the presence of an oxygen source to produce a product mixture comprising 4-chlorophthalic acid or 4-chlorophthalic anhydride, wherein an effective amount of at least one promoter is added to the reaction mixture after the reaction has proceeded to an intermediate stage of conversion to product mixture.
In still another embodiment the invention is a method for producing a product mixture comprising 4-chlorophthalic acid or 4-chlorophthalic anhydride which comprises oxidizing a substrate comprising 4-chloro-ortho-xylene, optionally in the presence of chlorotoluic acid, which comprises the steps of
(i) combining substrate in acetic acid solvent with at least one metal catalyst comprising cobalt, and optionally manganese, and heating in the presence of an oxygen source to a temperature in a range of between about 100xc2x0 C. and about 230xc2x0 C. at pressure in a range of between about 1300 and about 8300 kilopascals, wherein the molar ratio of substrate to the at least one metal catalyst is in a range of about 80-250:1;
(ii) adding, after the conversion to product mixture has reached about 35-65%, an effective amount of at least one promoter selected from the group consisting of N-hydroxyphthalimide, 4-chloro-N-hydroxyphthalimide, 3-chloro-N-hydroxyphthalimide, dichloro-N-hydroxyphthalimide, N-hydroxymaleimide, N-hydroxysuccinimide, 2-carboxyphenylhydroxamic acid, 4-chloro-2-methylbenzaldehyde, tetraethylammonium bromide, tetrabutylammonium bromide, and sodium bromide;
(iii) heating the reaction mixture under pressure of oxygen source to increase conversion to product mixture; and
(iv) isolating product comprising 4-chlorophthalic acid or 4-chlorophthalic anhydride.